The present invention relates to a transfer device provided in a printer employing an electrophotographic image transferring system.
An electrophotographic printer or the like is designed to perform the steps of forming a latent image by scanning the circumferential surface of a photoconductive drum with a laser beam carrying image data, adhering toner to the drum by means of a developing unit, transferring a toner image to recording paper by means of a transfer unit, and fixing the toner image thus transferred by thermocompression adhesion.
When humidity is high in the case of the conventional transfer charger, the transfer charger voltage should be set relatively low in order to obtain a appropriate printed (or, transferred) image. On the other hand, when humidity is low, the transfer charger voltage should be set relatively high in order to obtain an appropriate printed image. When the transfer charger voltage is set so that the image density becomes high either where the humidity is high or low, a faulty transfer arises in other environmental conditions.
When the voltage is preset so that a recording sheet is sufficiently charged in the case of high humidity, for instance, the charge amount will become insufficient when the actual humidity is low, whereas when the voltage is preset so that the sheet is sufficiently charged in the case of low humidity, the efficiency of transferring images to the paper will decrease when used in high humidity because the transfer voltage is unnecessarily high. In addition, imperfect cleaning will be unavoidable.